1. Field of the Invention
The present invention relates to mounting arrangements for semiconductor devices. In particular although not exclusively the invention is concerned with a mounting arrangement, or package, for an insulated gate bipolar transistor (IGBT).
2. Description of the Related Art
Present mounting arrangements for IGBTs until sea direct copper bonded (DCB) alumina substrate, which forms the base of the package as well as providing internal isolation. Other semiconductor devices and/or passive components may be mounted on the substrate, and the component-bearing, or populated, side of the substrate may be enclosed in a case and encapsulated with any of a variety of potting compounds.
Several methods are used for attaching connecting wires or bus-bars to copper areas on the substrate, which then make terminals on the outside of the package or module for the end user, one of which methods is to solder the wires or bus-bars directly to the copper area on the populated side of the substrate. With this method components can be mounted on the substrate and the sub-assembly completed before space-restricting external connections are made, which can be essential if space is limited, as is often the case with this kind of structure.
A problem which arises with such soldered joints is that the thermal expansion rate of the copper layer which forms part of the DCB "sandwich" is modified by the bond to the alumina, and is lower than that of free copper. A copper conductor soldered to the copper layer tends to expand and contract at its normal rate, and this results in fatigue and eventual failure of the solder joint.